In computer graphics, traditional real-time 3D scene rendering is based on the evaluation of a description of the scene's 3D geometry, resulting in the production of an image presentation on a computer display. Virtual Reality Modeling Language (VRML hereafter) is a conventional modeling language that defines most of the commonly used semantics found in conventional 3D applications such as hierarchical transformations, light sources, view points, geometry, animation, fog, material properties, and texture mapping. Texture mapping processes are commonly used to apply externally supplied image data to a given geometry within the scene. For example VRML allows one to apply externally supplied image data, externally supplied video data or externally supplied pixel data to a surface. However, VRML does not allow the use of rendered scene as an image to be texture mapped declaratively into another scene. In a declarative markup language, the semantics required to attain the desired outcome are implicit, and therefore a description of the outcome is sufficient to get the desired outcome. Thus, it is not necessary to provide a procedure (i.e., write a script) to get the desired outcome. As a result, it is desirable to be able to compose a scene using declarations. One example of a declarative language is the Hypertext Markup Language (HTML).
Further, it is desirable to declaratively combine any two surfaces on which image data was applied to produce a third surface. It is also desirable to declaratively re-render the image data applied to a surface to reflect the current state of the image.
Traditionally, 3D scenes are rendered monolithically, producing a final frame rate to the viewer that is governed by the worst-case performance determined by scene complexity or texture swapping. However, if different rendering rates were used for different elements on the same screen, the quality would improve and viewing experience would be more television-like and not a web-page-like viewing experience.